1. Field of the Invention
The present invention relates to a method for forming a film of a conductive substance, for instance a conductive high-polymer such as polypyrrole, polyaniline, polythiophene, polyfuran or the like, on a surface of a conductive body coated with an insulating film such as a dielectric oxide film and is suitably applied for manufacturing thermosensitive resistors, solid electrolytic capacitors, electroconductive films, electroconductive laminated films, gas sensors, printed circuit boards of aluminum oxide and the like.
2. Related Art
Conventionally, a chemical oxidation polymerization method has been used for forming a conductive high-polymer film on an insulating coating such as a dielectric oxide film having been formed by subjecting a conductive metal such as aluminum, tantalum or the like to a chemical conversion coating treatment. According to this method, the conductive high-polymer film is formed on the insulating coating by making a solution or gas of a monomer of a conductive high-polymer react to an oxidant.
Further, an electrolytic polymerization method has been used for forming a conductive high-polymer layer on an insulating coating or film. According to this method, a conductive thin film such as a conductive high-polymer film is formed on the insulating film by chemical oxidation polymerization beforehand and, thereafter, an electrolyzing current is supplied via an anode by contacting the same with the conductive thin film to form a conductive high-polymer film by the electrolytic polymerization.
These film forming methods have been applied for manufacturing solid electrolytic capacitors wherein conductive high-polymer films are used as an electrolyte. Namely, in the solid electrolytic capacitor, a conductive high-polymer film made of polypyrrole, polyaniline, polythiophene, polyfuran or the like is formed on an insulating dielectric oxide film using the chemical oxidation polymerization method or the electrolytic polymerization method.
Various manufacturing methods for solid electrolytic capacitors have been proposed in Japanese patent laid-open publications S63-173313, S64-32620 and S64-76713.
According to the method disclosed in the Japanese laid-open publication S64-76713, the dielectric oxide film formed on a conductive metal body is partially removed to short-circuit the metal body and a conductive thin film to be formed by the chemical oxidation polymerization method beforehand and, after forming the conductive thin film, a conductive film is formed by supplying an electrolytic current via the metal body. After forming the conductive thin film, the short-circuitted portion is insulated by making the same react to an oxidant and a deoxidant chemically or by applying a thermal energy using a laser beam or the like thereto.
Next, a conventional manufacturing method for solid electrolytic capacitors is explained concretely referring to FIGS. 6(a), 6(b) and 7.
At first, a dielectric oxide film 2 is formed on a surface of an aluminum body 1 composed of an sintered body of aluminum powder or an aluminum foil by electrolytic oxidation or air oxidation.
In the case of chemical oxidation polymerization, after applying a solution including an oxidant on the dielectric oxide film 2, the same is contacted to a solution including monomers of conductive high-polymer to form a conductive high-polymer layer 3 on the dielectric oxide layer 2 by the chemical oxidation polymerization. There are no problems in this forming process its self, but this method has such a disadvantage that it is impossible to obtain films having strength and electric properties enough for capacitors of this type.
Next, in the case of electrolytic polymerization, a capacitor element 6 with a lead wire 5 connected to the metal body 1 on which the conductive high-polymer layer 3 is formed by the chemical oxidation polymerization is dipped in an electrolyte 9, containing electrolytic supporting substances and monomers of a conductive high-polymer and a positive voltage is applied from a direct current power source 10 of constant voltage and current by contacting an external electrode 8 made of platinum or carbon with the conductive high-polymer thin film 3. The negative voltage of the DC power source 10 is supplied to a counter electrode 11 made of stainless steel or the like to perform an electrolytic oxidation polymerization.
According to the electrolytic oxidation polymerization, it is possible to obtain a conductive high-polymer film 4 of good quality but the current supplying method for forming the conductive high-polymer film 4 on the dielectric oxide film 2 is quite difficult.
As mentioned above, according to this current supplying method, the conductive high-polymer thin film 3 is formed on the dielectric oxide film 2 beforehand by the chemical oxidation polymerization and the external electrode 8 is contacted with the conductive high-polymer thin film 3 to supply an electrolytic current.
According to the above current supplying method, the current density becomes different due to the degree of contact of the external electrode resulting in that it is difficult to form a uniform film. Further, a significant loss of the electrolytic substance is inevitable since a conductive high-polymer film 4a is also formed on the external electrode 8.
Further more, as indicated by a reference numeral 4b in FIG. 6(b ) the dielectric oxide film 2 is partially damaged upon removing the external electrode 8 adhered to the capacitor element 6 by the conductive high-polymer film 4a from the capacitor element 6 forcibly and thereby a leakage current is increased. A reference numeral 7 indicates a cathode lead adhered onto the conductive high-polymer film 4 by silver paste applied thereto.
FIGS. 8(a) and 8(b) show another method for applying an electrolytic current.
As shown in FIG. 8(a), the bottom portion of the conductive high-polymer film 3 formed by the chemical polymerization is cut off to form a metal portion 1a exposed outwardly and an electrolytic current is supplied by a lead wire 5 to form a conductive high-polymer film 4 on the surface of the exposed metal portion 1a by the electrolytic oxidation polymerization, as shown in FIG. 9. Thereafter, as shown in FIG. 8(b), the conductive high-polymer film 4 is insulated using an oxidant or a deoxidant to form an insulating layer 13 (See the Japanese patent laid-open publication S64-76713). This method has disadvantages in that an uneven film is apt to be formed on the exposed metal portion and the partial insulating treatment (13) for the conductive high-polymer film 4 is troublesome especially in the case of small capacitors of tip type.